"I’ve never seen them washed up before. Twelve?" said Lindsey Mueller. "I was kind of concerned.”

That number has since increased to 15 dead rays found in four different ponds at the Don Edwards San Francisco Bay National Wildlife Refuge, according to Santa Clara County Park Ranger Brian Christensen.

The U.S. Fish and Wildlife Department says the bat rays are being inadvertently trapped as officials flood the salt ponds with bay water.

The process gradually encourages wildlife that thrive in habitats less salty, but a spokesman for the federal agency says bat rays and other fish sometimes get trapped, unable to handle initial salt levels and perhaps depleted oxygen levels.

“This is not an alarming development,” said Fish and Wildlife spokesman Doug Cordell. "If the occurrence was larger, we might change our management practice, but it’s not cause for alarm."